1. Field of the Invention
The present invention relates to an optical receiver and a method of detecting loss of optical signal of the optical receiver.
2. Description of the Related Art
In an optical receiver for receiving an optical signal transmitted in an optical transmission system, when the received optical signal is lowered in level or is lost, detection of loss of optical signal is performed for detecting such a state. It is very important to perform the detection of loss of optical signal with high precision in order to improve the reliability of data transmission. A conventional technology for performing the detection of loss of optical signal in the optical receiver is, for example, a technology described in JP 07-046203A. JP 07-046203 A discloses a method of generating an alarm for detecting loss of optical signal when the optical signal input to the optical receiver is lost in the optical transmission system.
FIG. 5 illustrates a structure of an optical receiver 9 according to the conventional technology. In the optical receiver 9 illustrated in FIG. 5, an optical signal transmitted through an optical transmission path is converted into an electrical signal by a photo electric conversion circuit 12. Upon receiving the optical signal, an optical signal power detector 14 detects a voltage corresponding to an optical current generated in the photoelectric conversion circuit 12. A comparator 18 compares the detected voltage with a threshold voltage set by a threshold voltage generator 16. When a voltage level of the optical signal is low, a first determination signal (S1) is output. The threshold voltage generator 16 sets two different threshold values to the comparator 18 to provide a hysteresis characteristic as illustrated in FIG. 6A in order to determine the voltage level of the optical signal. That is, the voltage level of the optical signal is initially compared with a first threshold value (TH1). When the voltage level becomes smaller than the first threshold value, the first determination signal (S1) is output. Once the first determination signal becomes an output state, a second threshold value (TH2) larger than the first threshold value is set by the threshold voltage generator 16 so that the output of the first determination signal is not cancelled before the voltage level becomes larger than the second threshold value TH2. In FIG. 6A, Vs1 indicates a voltage of the first determination signal.
The electrical signal obtained by conversion by the photoelectric conversion circuit 12 is amplified by an amplifier 20. A timing extraction circuit 22 generates a clock signal based on an output of a phase locked loop (PLL), which is synchronized with a frequency component of the amplified electrical signal. A discriminator 24 obtains digital data (received data) from the amplified electrical signal based on the clock signal generated by the timing extraction circuit 22.
As illustrated in FIG. 6B, when an amplitude voltage of the generated clock signal is smaller than a reference threshold value (TH3), the timing extraction circuit 22 outputs the second determination signal (S2). In FIG. 6B, Vs2 indicates a voltage of the second determination signal. The first determination signal and the second determination signal are input to an optical signal loss detector 26 (OR circuit). When one of the first and second determination signals is output, an optical signal loss state is determined and thus a signal of detection of loss of optical signal (S3) is output. In the optical receiver 9 operating as described above, when an optical signal having a pattern as illustrated in an upper part of FIG. 7A is received, the signal of detection of loss of optical signal (S3) has a hysteresis characteristic as illustrated in a lower part of FIG. 7A. In FIG. 7A, Vs3 indicates a voltage of the signal of detection of loss of optical signal.
According to the conventional technology described above, there is a case where the hysteresis characteristic of the signal of detection of loss of optical signal is not effectively exhibited based on the first determination signal depending on the pattern of the received optical signal. That is, when an optical signal having a pattern as illustrated in an upper part of FIG. 7B is received with respect to each of the threshold value of the first determination signal and the threshold value of the second determination signal in the optical receiver 9, the second determination signal is output, and hence the detection of loss of optical signal is temporarily performed. However, the first determination signal is not output. Therefore, even when the detection of loss of optical signal is performed, the output characteristic of the detection of loss of optical signal as illustrated in a lower part of FIG. 7B is obtained. Thus, according to the conventional optical receiver 9, there is the case where the hysteresis characteristic of the detection of loss of optical signal is not effectively exhibited based on the first determination signal. As a result, there is a case where the detection of loss of optical signal is not performed even when the quality of the received optical signal is not sufficiently stable.